1. Field of the Invention
The invention relates to proximity sensors and in particular to a thermally self-adjusting mount for controlling the position of such sensor.
2. Description of the Prior Art
Turbines and similar machines may include one or more blade rows connected to a rotating shaft member. The blades typically are of a complex design resulting in a multiplicity of vibrational modes. It has been found to be desirable to provide apparatus for monitoring blade vibration to anticipate and avoid damage to the turbine due to undesirable vibrational modes. Such apparatus is disclosed in prior U.S. Pat. Nos. 4,518,917 and 4,573,358, the entireties of the disclosures of which are hereby specifically incorporated by reference.
Generally speaking the apparatuses disclosed in the '917 and the '358 patents employ proximity sensors for monitoring blade vibration. Sensors of the eddy-current type have been found to be particularly suitable. Such sensors are well known and operate on the principle that the impedence of an AC-excited electrical coil is subject to change as the coil is brought in close proximity to a metal object. A suitable sensor which may be adapted for use in connection with the present invention is described in prior U.S. Pat. No. 4,563,643, the entirety of the disclosure of which is also hereby specifically incorporated herein by reference.
As illustrated particularly in the '358 patent identified above, the vibration of turbine blades may be monitored on-line by radially mounted blade tip sensors placed circumferentially about the bladed disk. In conducting such monitoring, it has been found that the gap between the blade tip and the sensor is a critical parameter in enabling the sensors to resolve the vibrating motion of individual blades. The minimum gap is determined by a number of physical considerations such as (1) the degree of permanent ovality of the sensor mount structure; (2) the irregularity of the blade length; (3) the difference in thermal expansion rates of the blades and sensor mount structure as the turbine is taken from equilibrium at ambient temperature to a steady state operating temperature; and (4) the non-uniform radial growth of the sensor mount structure due to gravity and asymmetrics induced by support points and gravity. The first of these considerations may be satisfied, if necessary, by initial sensor positioning and trimming of blade tips, respectively. Accordingly, in connection with the present invention, these two considerations are assumed to be minimized by techniques which are well known in the turbine art. Thus, the present invention deals with considerations (3) and (4).
The difference in the rate of thermal expansion of the blades and the sensor mount structure imposes a lower limit on blade tip sensor gap that restricts the use of magnetic sensors and turbine blade vibration monitoring systems. This results because the ligher blades reach operating temperature before the massive sensor mount structure. However, it is known for purposes of monitoring, that the gap between the blade tip and the sensor should generally be minimized to eliminate cross talk between the closely spaced steam turbine blades and to maximize the slope of the sensor signal as the blade passes.
The general chronology of the width of the gap during normal turbine start-up is as follows.
With the turbine at ambient temperature, a gap, g, exists between the blade tips and sensors. The turbine is heat soaked causing the blades to warm quickly and expand much faster than the massive sensor mount structure, thus reducing the gap to some small value g' which is much smaller than g. It is this initial start up period which determines the value of g and also the value of the minimum gap if considerations (1) and (2) mentioned above have in fact been eliminated. The heavier sensor mount structure then slowly heats up causing the gap to grow until the gap between the sensor and the blades is again the initial value g. The minimum running gap is thus determined by the maximum thermal disparity between the light blades and the massive sensor mount structure.
The foregoing considerations are well known in the turbine art and the linear displacement and/or position of each point in the turbine structure during initial start-up, steady state operation and shut-down is readily determinable by known empirical and comparative techniques. Manifestly, the initial gap between the blade tips and the sensors must be sufficient so that the blades may achieve their maximum lengths due to heat expansion before there is any linear movement of the turbine housing itself. Thereafter the housing expands outwardly and the initial gap is reestablished.